A growing worldwide need for fresh water for potable, industrial, and agricultural uses has led to an increase in the need for purification methods that use seawater, brackish water, or other elevated salinity water as sources. The purification of high salinity water through the removal of dissolved solids, such as salts, has been accomplished in several ways including distillation and reverse osmosis (RO). These methods start with a pretreated feed of seawater or other brackish water and then purify (e.g., desalt) the water to a level that is suitable for human consumption or other purposes. While seawater and often, brackish water, is a plentiful starting material, the energy required to convert it to drinking water using present RO or distillation techniques is often cost prohibitive.
The ocean provides a limitless source of water if efficient desalination techniques can be developed with low environmental impact. While equipment cost can be high, the greatest continuing expense in desalting high salinity water is energy. A small improvement in energy efficiency can result in significant cost savings due to the large volumes of water that are typically processed by desalination systems.
For example, the energy required to produce potable water from seawater by the RO process is comprised primarily of the energy that is required to overcome the osmotic pressure of the seawater, along with pressure loss inefficiencies during processing. Because both RO permeate and RO wastewater (often 70% of the total water fed to the system is lost to waste) must be pressurized, RO energy consumption is much higher than the theoretical thermodynamic minimum for desalination. Expensive mechanical pressure recovery devices are commonly needed in an attempt to recover some of the lost energy required for pressurization.
Seawater typically contains about 20,000-40,000 ppm (mg/l) of total dissolved solids (TDS), and brackish water sources can contain from 2,000 ppm to as much as 20,000 ppm TDS. These dissolved solids include a variety of monovalent, divalent, polyvalent, and/or multivalent salts or species, with sodium chloride typically forming about 75% or more of the total solids content.
While evaporative methods such as distillation have been traditionally used to produce potable water, these methods typically require even greater amounts of energy than do systems utilizing reverse osmosis techniques. Further these systems typically utilize complicated heat recovery techniques to improve energy efficiency. Because RO or distillation based processes operate at elevated pressures or temperatures, and because high salinity water is very corrosive, exotic metals and alloys are needed to withstand the operation conditions, and thus the need to add complicated equipment in these processes to save energy further increases the initial cost of the equipment and greatly decreases the equipment reliability.
Reverse osmosis techniques can be effective at removing ionic compounds from seawater. However, one serious drawback of RO systems is that RO membranes selectively reject non-monovalent or multivalent salts to a higher extent than monovalent salts. Thus for purification purposes in applications such as agriculture, where most divalent ions such as calcium and magnesium are actually beneficial for irrigation use, these ions are rejected selectively, resulting in higher than needed operating pressures, increased potential for membrane fouling and scaling, and/or loss of valuable minerals for use in crop production.
The difference in osmotic pressure between seawater containing over 3.5% solids and potable water at less than, 1,000, or less than 500 ppm, TDS dictates that high pressures be used to produce a permeate of potable quality simply to overcome the thermodynamic free energy potential. In practice, since seawater is usually processed at elevated water recoveries to reduce pretreatment cost by reducing the amount of water that needs to be effectively prepared for treatment, the required osmotic pressure is even higher than needed to process seawater at 3.5% solids. For example, pressures utilized in RO systems are typically greater than 800, 900, or even 1,000 psi and for practical considerations of high pressure operation, corrosion resistance, avoidance of energy losses, and prevention of scaling due to divalent selectivity and silica rejection, are limited in water recoveries (the ratio of product water production to total water production) of around 30% to 40%. This limitation results in a very high incremental cost of pretreatment and water use for RO systems when it is considered that a change in water recovery from about 67% to about 33% results in a doubling of pretreatment equipment costs and a doubling of overall water consumption for a given pure water need. Recent advances in RO membranes and in energy reuse techniques have lowered the power consumption of producing potable water using RO systems to about 7 to 14 kwh per 1,000 gallons (14 kwh/kgal) of water produced.
Alternative techniques using a combination of processes have also provided for lower energy consumption in the conversion of seawater to fresh water. For example, two-pass nanofiltration systems have been shown to be capable of producing potable water using a total working pressure of about 750 psi; about 500 psi in a first stage and about 250 psi in a second stage. Because energy usage relates to operating pressure, a total working pressure of about 750 psi provides for a more energy efficient system compared to a typical RO system operating at a pressure greater than 800 psi. See, for example, the teaching of Vuong in U.S. Patent Publication No. US2003/0205526, which is incorporated by reference herein.
In another method used to produce fresh water from seawater, nanofiltration techniques are used in conjunction with either RO or flash distillation techniques. See, for an example, the teaching of Hassan in U.S. Pat. No. 6,508,936, which is incorporated by reference herein.